A radiation detector of the gas-filled proportional counter type, includes a cathode and anode and a gas lying between them. A radiator particle such as a photon of X-ray or gamma ray radiation entering the chamber, ionizes gas atoms, with the number of ionizations being proportional to the photon energy. The electrons formed in this process are accelerated towards the anode. When the electrons gain sufficient kinetic energy they too will ionize gas atoms and start an ionization avalanche in the strong electric field near the thin wire anode. The net number of electrons which reach the anode wire (and hence the charge) can be closely proportional to the photon energy. The number of electrons generated by a photon of a particular energy depends upon the electric field in the avalanche region that extends within a few tens of microns around the anode. In order for the same number of avalanche electrons to be generated for a photon of given energy passing through any region of the chamber that lies between the cathode and anode, it is important that the electric field gradient be the same in every direction extending radial to the wire anode in the small avalanche region around the anode.
A uniform electric field gradient around a wire anode of a radiation detector, can be achieved by utilizing a wire anode that extends along the axis of a cylindrical cathode that surrounds the wire. Gas filled proportional counters of this construction are utilized as standard devices for measuring radiation intensity, such as in Mossbauer scattering of a gamma ray beam transmitted through a thin foil specimen. However, the simple cylindrical coaxial radiation detector design cannot be easily utilized in certain applications such as in detecting backscattered radiation wherein the radiation source must be located in approximately the same direction from the specimen as is the detector.
A detector design useful in backscatter radiation detection, employs a toroidal detector. Radiation can pass from the source through the hole in the torus to the sample, and the backscattered radiation can be detected over a wide solid angle by the toroidal detector. A detector of this general configuration is shown, for example, in U.S. Pat. No. 3,011,060 by Dorembosch et al. wherein a thin wire anode extends along the minor axis of the torus and the cathode forms the walls of the torus. However, a simple detector of this configuration produces an output that is dependent upon the particular location within the torus through which the radiation passes, and therefore is not a high resolution proportional counter. An attempt has been made to avoid the non-uniformity of the electric field gradient immediately surrounding the anode, by surrounding the anode wire with a biased cylindrical grid. While such a grid can improve the performance of the detector to a level nearly compatable to that of a cylindrical proportional counter, it requires careful alignment of the grid with respect to the anode, which results in a delicate and costly detector. A wide angle proportional counter with good resolution and which provided a hole around the detection region to enable backscatter radiation detection, and which was of relatively simple and rugged design, would be of considerable value in radiation detection, as in back scatter Mossbauer effect spectroscopy.